Abstract
This paper analyzes the nozzle start-up of the von Kármán Institute Longshot, an impulse hypersonic gun tunnel, using a combination of the Lagrangian L1D code (used to model the initial compression process of the test gas into a reservoir by an inertial piston) and unsteady Reynolds-averaged Navier–Stokes simulations to model the flow expansion through the appended nozzle. The influence of initial nozzle flow conditions on the duration of the start-up is investigated based on various simulated experiments, and it is verified that the shortest establishment time is obtained for the lowest initial pressure. The applicability of the numerical scheme is proved through a comparison of the inlet and outlet total and static conditions against experimental results at relevant conditions. Besides, the right running and left running shocks constituting the start-up shock system are tracked through the nozzle, and their propagation velocities are reported. Momentum and thermal boundary-layer establishment times are reported based on the temperature and velocity profiles predicted at the nozzle exit. Finally, the flow establishment time is compared against several criteria available in the literature. Additionally, new establishment and quasi-steady behavior criteria are presented, derived from the freestream static pressure and temperature evolution.
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